Electrodeposition of rutenium

ABSTRACT

RUTHENIUM COMPLEX COMPOUNDS ARE PRODUCED IN WHICH THE ANION HAS THE GENERAL FORMULA (RU2N(H2O)XYY)ZWHEREIN Y IS A CHLORO OR BROMO GROUP, X EQUALS 2, 3 OR 4, X+Y EQUALS 10 AND Z EQUALS 5-X, AND THE CATION IS MOST ADVANTAGEOUSLY MONOVALENT, FOR EXAMPLE, AMMONIUM, LITHIUM, SODIUM OR POTASSIUM. A PLATING BATH IS PREPARED USING THE RUTHENIUM COMPLEX COMPOUNDS IN AQUEOUS ACIDIC SOLUTION AND THE PLATING BATH IS EMPLOYED IN THE ELECTRODEPOSITION OF RUTHENIUM.

United States Patent Int. Cl. C23b 5/24 US. Cl. 204-47 7 Claims ABSTRACT OF THE DISCLOSURE Ruthenium complex compounds are produced in which the anion has the general formula [Ru N(H O) Y wherein Y is a chloro or bromo group, at equals 2, 3 or 4, x+y equals 10 and z equals 5 x, and the cation is most advantageously monovalent, for example, ammonium, lithium, sodium or potassium. A plating bath is prepared using the ruthenium complex compounds in aqueous acidic solution and the plating bath is employed in the electrodeposition of ruthenium.

The present invention relates to novel ruthenium compounds, to methods for the preparation thereof, and particularly to the use of such compounds in the electrodeposition of ruthenium.

Ruthenium has attractive properties as an electrical contact material (a low contact resistance at temperatures up to 600 C. 1100 F.) and resistance to welding at these temperatures). In the electrodeposited condition ruthenium has a high hardness of from 900 DPN to 1300 DPN. Such electrodeposits of ruthenium are valuable as coatings for electrical contacts, for example, those in reed switches, and for other surfaces requiring good resistance to wear.

In one process known to the art, ruthenium is electrodeposited cathodically from a fused electrolyte. In this process the electrolyte consists of a fused mixture of sodium and potassium cyanides that has been charged with ruthenium by alternating current electrolysis be tween ruthenium electrodes. This process gives satisfactory coherent coatings of up to 0.005 inch thick but has the disadvantage that a temperature of about 500 C. must be used to maintain the bath in a molten condition.

Ruthenium has also been electrodeposited commercially in processes employing a ruthenium nitrosyl sulfamate electrolyte. This electrolyte has produced deposits having satisfactory characteristics, but it operates at a relatively low cathode efficiency, and it is difiicult to prepare reproducibly on a large scale, that is, in baths containing more than 200 grams of ruthenium. Too, this process (and others) is subject to sludge deposition during use. In any event, as far as we are aware, etforts to improve the cathode efiiciency and the batch size of plating baths using the ruthenium nitrosyl sulfama-te electrolyte have not been entirely successful.

It as now been discovered that a novel ruthenium complex compound can be made which, in aqueous acidic solution, provides an electroplating bath which can be produced in batches which contain relatively large amounts of ruthenium and from which ruthenium may be electrodeposited with a relatively high cathode efiiciency.

It is an object of the present invention to provide a novel process for the electrodeposition of ruthenium.

It is another object of this invention to provide a novel plating bath for the electrodeposition of ruthenium with a relatively high cathode efiiciency.

3,576,724 Patented Apr. 27, 1971 Still another object of this invention is the preparation of novel ruthenium complex compounds which are soluble in water.

Other objects and advantages will become apparent from the following description.

In accordance with the invention, ruthenium is electrodeposited from an aqueous acidic solution of a soluble ruthenium complex compound. The plating solutions are prepared using novel ruthenium coordination compounds in which ruthenium is present in an anionic complex of the following general formula:

wherein each Y is a chloro or bromo group; x=2, 3 or 4; x+y= l0, and z='5 x.

In the above complex the coordination number of ruthenium is 6. The nitrogen atom satisfies one position for each ruthenium atom and the remaining 10 positions are filled by laquo groups and chloro or bromo groups. Thus x+y is 10. The ruthenium atoms are tetravalent and therefore the total positive charge due to ruthenium is 8. Nitrogen has a negative charge of 3. If the remaining ten ligands were all chloro or bromo groups, i.e. anions having in total a negative charge of 10, then the aggregate negative charge on the complex anion would be 5. However, some ligands are neutral aquo groups and the negative charge z is reduced by 1 for each aquo group present. Tus z=5x.

Generally speaking, the compounds in accordance with the invention advantageously contain anions having the formula: [Ru N(H O) Y The compounds are, most advantageously, salts of monovalent cations, in particular ammonium, lithium, sodium or potassium salts.

For use in the electroplating baths which form part of this invention suitable compounds are ammonium salts of the general formula [Ru N(H O) Y (NH The ammonium salt of the bis-aquo-chloro complex is prepared by heating an aqueous solution of ruthenium chloride with excess sulphamic acid for long enough to hydrolyze the sulphamate. The ammonium salt of the complex is then obtained from the reaction mixture. To obtain the solid compound it is usually necessary first to saturate the solution by evaporation followed by cooling, preferably to about 0 C., e.g. with ice or ice-cold water. The compound may then be separated by filtration and should be washed with ice-cold water to free it from highly water-soluble impurities, such as sulphuric acid and ammonium sulphate and chloride.

Ruthenium chloride is often supplied commercially in hydrochloric acid solution. Care should be taken to avoid excessive amounts of hydrochloric acid in the initial reaction mixture since this can lead to the formation of compounds other than the desired complex. We find it advantageous when using commercial solutions of ruthenium chloride in hydrochloric acid first to evaporate the solution to obtain a concentration of g. of ruthenium in 300 ml. of acid solution.

However, after reacting the ruthenium chloride with sulphamic acid in aqueous solution it is advantageous to add some hydrochloric acid to obtain the best yield of the aquo-chloro complex. This addition of hydrochloric acid helps to prevent bisulphate groups, for example, from entering the complex as ligands in place of the chloro groups. Such liganding of bisulphate groups may occur in the absence of suflicient chloride ion since high concentrations of ammonium bisulphate and sulphuric acid may form in the reaction mixture. For this purpose at least 3 ml. of concentrated hydrochloric acid should be added for each gram of ruthenium. The addition is best made 3 after concentrating the reaction mixture and allowing it to cool since otherwise upleasant fuming may occur.

In the preparation of the compound, the use of excess amounts of sulphamic acid may lead to the production of complexes with slightly different physical characteristics; for example, steric isomers. This is illustrated by Examples I and III hereinafter in which was prepared (Samples A and B), using different amounts of excess sulphamic acid. In preparing the aquo-chloro complex we prefer to use from 4 to 9 molecular proportions of sulphamic acid for each atomic proportion of ruthenium. Also, we have found that when the aqueous reaction mixture is prepared from solid, hydrated ruthenium chloride rather than from commercial ruthenium chloride solutions, better yields are obtained with smaller excesses of sulphamic acid, e.g., 4 to molecular proportions of sulphamic acid for each atomic proportion of ruthenium.

Examples of the preparation of the novel complex ruthenium compound [Ru N(H O) C1 (NH contemplated herein are now given:

EXAMPLE I An aqueous solution of ruthenium chloride containing 50 grams (g.) of ruthenium in 2000 cc. of solution was refluxed with 300 g. of sulphamic acid at the boiling point for 48 hours. The solution was then concentrated to 500 cc. and cooled to room temperature. The solution was then further cooled to about 0 C. with ice cold water for about 3 /2 hours and the complex was separated (60% yield) as dark red crystals which were filtered, washed with ice cold water and dried. On further evaporation of the filtrate and cooling a further 30% of the product separated, giving a total yield of 90% (Sample EXAMPLE II An aqueous solution of ruthenium chloride containing 50 g. of ruthenium in 2000 cc. of solution was refluxed with 300 g. of sulphamic acid at the boiling point for 48 hours. The solution was then concentrated to 500 cc. and cooled to room temperature. 200 cc. of concentrated hydrochloric acid were added to the solution which was then cooled with ice cold water for about 3 /2 hours. The complex separated as dark red crystals (about 85% yield) and was filtered, washed with ice cold water and dried. 300 cc. of distilled water were added to the filtrate and the solution was evaporated until bumping occurred. On cooling the solution to room temperature a further yield of product was obtained.

EXAMPLE III An aqueous solution of ruthenium chloride containing g. of ruthenium in 1000 cc. of solution was refluxed with 100 g. of sulphamic acid at the boiling point for 15 hours, then evaporated until bumping occurred, cooled to about 0 C. for between 3 and 4 hours and filtered. The complex was obtained (80% yield) as a reddish-brown crystalline solid, which was washed with 80% water-acetone. The product, designated Sample B, was found to be rather more soluble in water than Sample A.

From the aquo-chloro complex other compounds may be prepared. The following example describes the preparation of the corresponding aquo-bromo complex, [Ru (H O) Br ](N'H EXAMPLE IV A solution of 15 g. of [Ru N(H O) Cl ](NHs; in 100 ml. of water was refluxed for 3 hours with 50 ml.

of hydrobromic acid (analytical reagent grade of specific gravity 1.47). The reaction mixture was then evaporated 4 to 50 ml., cooled to room temperature and the resultant brown crystalline product, [Ru N(H O) Br8l (NH was filtered and washed with ice cold water. The yield was about While the examples given are confined to the production of ammonium salts of the complexes, other salts may easily be prepared using the ammonium salts as starting materials. The most convenient way of doing this is by conventional ion-exchange techniques. For instance, the corresponding hydrogen, sodium, potassium and other alkali metal salts may be prepared by this method. Other chemical processes are also capable of producing such salts. However, when a product of high purity is required, a method involving ion-exchange techniques is advantageous. Such a method is illustrated by the following example.

EXAMPLE V A solution of 5 g. of [R11 N(H O) Cl ](NH in ml. of water was poured through an ion-exchange column at a rate of 2 to 5 drops per second. The column had a capacity of 270 ml. and contained a cation-exchange resin (sold under the trademark Zeocarb 225) in the acid form. Distilled water was used to wash the remaining solution from the column and the hydrogen salt of the complex was obtained as a dark reddish-brown deliquescent solid by evaporating the solution to dryness.

While the ammonium salts are much preferred for use in plating baths, particularly because of their case of preparation, other salts such as the lithium, sodium or potassium salts mentioned above herein, may have other specific advantages, e.g., have greater solubility, thus enabling a higher ruthenium concentration to be employed in the bath. Moreover, both the ammonium and other salts of a particular complex may be used as the starting material for the preparation of another complex containing different ligands. Such a preparation has been described in Example IV. Such substitution of ligands is well known in coordination chemistry.

In respect of depositing hard, wear resistant ruthenium surfaces, We find that ruthenium can be satisfactorily electrodeposited from acidic aqueous solutions of our novel ruthenium compounds in which ruthenium is present in the aforedescribed anionic complex of general formula [Ru N(H O) Y Baths for the electrodeposition of ruthenium can be obtained by dissolving the ruthenium complex in water and adding an appropriate acid to adjust the pH to 4 or less. The acid used to effect the adjustment of pH obviously should not cause the breakdown of the complex. For example, when the complex used is of the formula: [Rll2N(H2O)2Cl8] (NH the addition of hydrochloric acid or sulphuric acid is suitable. Similarly, when the complex is of the formula [Ru N(H O) Br (NH the addition of hydrobromic 'acid is then suitable.

If the pH of the bath solution is too low, the deposition of ruthenium takes place at a low cathode current efficiency, while if the pH is too high the resulting ruthenium deposit is black. Increasing the pH still further leads to the formation of a brownish deposit on the cathode that is not metallic ruthenium. In practice the pH should be from 0.5 to 4 and beneficially it is from 1.5 to 2.5. In making up the bath, ammonium salts, e.g., ammonium chloride or ammonium bromide, may be added to increase the conductivity.

The plating bath conveniently contains from 5 to 20 grams per liter (g./l. hereinafter) of ruthenium. With lower ruthenium concentrations frequent replenishment of the bath is required. Although higher concentrations of ruthenium up to the limit of solubility of the compounds may be used, there is no practical advantage in doing so. Moreover, losses of ruthenium from the bath by drag-out are increased, and the capital cost of the bath becomes unnecessarily high. Agitation of the bath is desirable at ruthenium contents exceeding 20 g./l.

Suitable insoluble anodes for use in plating from the baths are of platinum and platinized titanium.

The temperature of the bath is advantageously from 50 C. to 70 C. or 75 C., since plating takes place more slowly at lower temperatures, while higher temperatures lead to excessive losses by evaporation. Moreover, at temperatures below 50 C. the baths can only be operated within a limited range of current densities, since the cathode etficiency is found to fall off rapidly as the current density is increased above 1 ampere per square decimeter (A./dm. Thus, if only a slow rate of deposition is required the baths may be used at temperatures down to room temperature, but the current density should not exceed 1.5 A./dm. at room temperature if satisfactory deposits are to be obtained. At 50 C. plating may be performed with current densities in the range of 0.5 to 4 A./dm. and at 70 C. in the range of 0.5 to 10 A./dm. provided that at current densities above 4 A./dm. the pH of the solution is in the range 0.5 to 1. At current densities lower than 0.25 A./dm. the rate of deposition is very low, while higher current densities than the maxima indicated lead to loss of efficiency and ultimately to burning of the deposit. Provided the current density does not exceed 2 A./dm. at 50 C. or 4 A./dm. at 70 C., the cathode efficiency of a bath containing 10 g./l. of ruthenium is generally at least 75%, calculated for tetravalent ruthenium.

The baths are readily replenished by the addition of the solid ruthenium complex compound. The pH tends to decrease during operation of the bath, but it is readily adjusted by the addition of dilute ammonium hydroxide solution. After prolonged use, the solid compounds may be recovered by evaporating the solution to low volume and cooling. The solid is then washed free from unwanted salts, e.g., ammonium salts, with ice-cold water, and redissolved for further use.

The operation and performance of plating baths within the invention are illustrated by the following examples.

EXAMPLE VI Baths were prepared by dissolving the aquo-chloro complex [Ru N(H Cl ](N'H (taken from Sample A) in water to give ruthenium concentrations of g./l. and g./l. To each bath 5 g./l. of ammonium chloride was added to increase the conductivity and the pH was adjusted to pH 2.5 by the addition of hydrochloric acid. The baths were then used at 50 C. to deposit ruthenium on copper cathodes that had been given a flash coating of gold, using platinum anodes. The current density was varied, and the cathode efficiency and plating rate observed under each set of conditions are shown in Table I.

TAB LE I 5 g.ll. Ru 10 g./l. Ru

Deposit Deposit Current Cathode thickness Cathode thickness density efificiency in mins. efiiclency in 20 mins. a./dm. (percent) (microns) (percent) (11110!0115) EXAMPLE VII A bath was prepared again by dissolving the aquochloro complex [Ru N(H O) Cl ](NH (taken this time from Sample B) in water to give a ruthenium concentration of 10 g./l. and used, after adjustment of its pH to 2.5 by addition of hydrochloric acid, to deposit ruthenium on gold-flashed copper cathodes using .a platinum anode. At a temperature of 50 C., and current density of 1.5 A./dm. the plating rate was 9 microns/hour and the cathode current efficiency was 80% 6 EXAMPLE VIII A bath was prepared by dissolving sutficient of the aquo-bromo complex of formula in water to give a ruthenium concentration of 10 g./l. Dilute hydrobromic acid was added to adjust the pH to 2.5. The bath was then used at 70 C. to deposit ruthenium on gold-flashed copper cathodes, using platinum anodes. At a current density of 1.0 A./dm. a deposit of ruthenium 2.5 microns thick was obtained in 20 minutes. The cathode current efliciency was close to During electrodeposition, 'With some combinations of bath composition and operating conditions it is found that evolution of undesirable products may occur at the anode. In particular, this tendency is favored by high concentrations of ammonium chloride or bromide accumulated in the bath, a low pH, and a high anode current density. The product evolved, if any, depends on the bath composition. For instance, from baths prepared from compounds containing [R-u N(H O) Cl anion, nitrogen trichloride and intermediate products, possibly mixed with other compounds, are evolved. The amounts of nitrogen trichloride and other compounds liberated are small and under the conditions of operation of the baths, particularly at the higher temperatures, tend readily to decompose.

A similar condition may arise when electrodepositing ruthenium from baths containing the corresponding bromo-complex anion. However, any nitrogen tribromide which may form at the anode is unstable and any product evolved consists predominantly of bromine.

This evolution of undesirable products at the anode may be suppressed or even wholly eliminated by adding sulphamic acid or a soluble sulphamate to the bath. If desired, the evolution at the anode can be suppressed by the addition of other mild reducing agents compatible with the bath, e.g., ethanol. However, great care must be taken in the choice of such reducing agents since they may themselves affect the bath or be oxidized to undesirable compounds. For example, if urea were used for this purpose it could possibly enter the anionic complex and co-ordinate with the ruthenium, replacing some other ligands, Also, for example, ethanol may be oxidized, first to acetaldehyde and then to acetic acid and it may be desired to avoid such oxidation products. For these reasons we prefer to add sulphamic acid or a soluble sulphamate. Most preferably, we add ammonium sulphamate, since the addition of sulphamic acid lowers the pH, which must then be restored by the addition of ammonium hydroxide, so that the net effect is the same. Sulphamates of cations that might adversely affect the plating, must, of course, be avoided. A further advantage of adding sulphamate is that it increases the conductivity of the bath.

Even a small amount of sulphamate is effective in suppressing lthe undesirable evolution. However, the sulphamate is progressively lost from the solution both by hydrolysis and also by a reaction with the anode products formed during electrolysis which appears to lead to a loss proportional to the amount of ruthenium deposited. We prefer to make an initial addition of at least 10 g./l. of ammonium sulphamate. Thereafter, we prefer to add at least 2 g. of ammonium sulphamate for each gram of ruthenium deposited from the bath. It will be appreciated that it is the usual practice to replenish plating baths at intervals when the ruthenium content has been depleted by about 20%, and it is convenient to introduce further ammonium sulphamate together with the ruthenium compound used to replenish the bath. Advantageously, the proportions are from 1 to 2 parts by weight of ammonium sulphamate for each part of ruthenium in the ruthenium compound added.

It is unnecessary to add initially an amount of ammonium sulphamate exceeding the amount of ruthenium 7 in the bath, but the introduction of greater amounts, e.g. up to 50 g./1., does not appear advantageous.

Baths to which sulphamate is added may be operated Without the evolution of undesirable anode products under a wider range of conditions than sulphamate-free baths. In particular they may be used at a higher acidity than pH 1, e.g. down to pH 0.5 or lower, though this may not be desirable since it may lead to corrosion of the surface being plated. Moreover, under very acid conditions the rate of hydrolysis of the sulphamate is increased and more frequent additions of sulphamate are required. Gen erally speaking, the bath compositions and operating conditions for sulphamate-free baths are also satisfactory when the baths contain sulphamate. An advantageous bath composition contains initially 12 g./l. of ruthenium added as the aquo-chloro complex,

and 12 g./l. of ammonium sulphamate, has a pH of 1.5 to 2.0 and is operated at 70 C.

EXAMPLE IX By way of example, a bath containing 5 g./1. of ruthenium was prepared by dissolving some of the aquochloro complex (Sample A) in distilled water that had been acidified to pH 2.5 to 3 by means of hydrochloric acid. The temperature was raised to 70 C. and the pH adjusted to 1.5 to 2 by means of a further addition of hydrochloric acid. Ruthenium Was deposited from this bath at 70 C. on to a copper cathode that had been given a flash coating of gold, using a platinized titanium anode, a cathode current density of 1.0 A. dm. and an anode current density of 0.2 A./dm. The bath was periodically replenished with ruthenium when its ruthenium content had fallen to 4 g./l., by the addition of the solid aquo-chloro complex, and the pH was maintained at 1.5 to 2 by means of dilute aqueous ammonium hydroxide. When about 3 g./l. of ruthenium had been deposited, evolution of nitrogen trichloride at the anode became noticeable. An addition of 10 g./l. of ammonium sulphamate was then made to the bath, which immediately suppressed the nitrogen trichloride evolution. No further evolution was observed when plating was continued until a total of 7 g./l. of ruthenium had been deposited, when the run was discontinued. During this further plating, one gram of ammonium sulphamate was added to the bath for each gram of ruthenium added during its periodical replenishment.

EXAMPLE X TAB LE II Cathode current Cathode Plating rate Temperature density efliciency (microns/ (C.) pH (a. Idmi) (percent) hour) No evolution of nitrogen trichloride was observed at the anode under any of these sets of conditions.

The deposits obtained from the baths of the invention are bright up to a thickness of about 2-3 microns. As plating proceeds to greater thicknesses, surface cracking becomes apparent, and at still greater thicknesses the deposits a-re matt and grey. If desired, small amounts of 'conventional organic stress-relieving agents that are compatible with the bath may be added. The high cathode efiiciency of the baths results in substantial freedom from gassing at the cathode, and the deposits are therefore almost free from streaking and pitting.

While we have described the complex compounds as being compounds containing an anionic complex of the general formula [Ru N(H O) Y it is believed that they are nitrogen-bridged complex compounds of ruthenium, the anion of which has the following structure:

in which L represents the aquo, chloro or bromo ligands.

There are several sources of evidence which supports this structure.

Analytical results for the bis-aquo-chloro complex [Ru N(H O) C1 (NH are in good accord with the results calculated for the compound in the form of an N-bridged complex, as may be seen from the following Table III:

TABLE III Percentage composition Analytical Calculated NH; (ammonium) 11% (other than ammonium).--

u E20 (by difference) Debye-Scherrer camera. The major diifraction peaks were as follows:

D, A.: Intensity 7.8 80

As will readily be seen from the proposed structure, it is possible for steric isomers to exist, which may differ slightly in physical characteristics.

Although the present invention has been described in conjunction with preferred embodiments, it is to be understood that modifications and variations may be resorted to without departing from the spirit and scope of the invention as those skilled in the art will readily understand. Such modifications and variations are considered to be within the purview and scope of the invention and appended claims.

We claim:

1. A bath for the electrodeposition of ruthenium comprising an acidic aqueous solution of a complex ruthenium compound having a cation selected from the group consisting of ammonia, hydrogen, lithium, sodium and potassium, the ruthenium being present in the form of an anionic complex of the formula [Ru N(H O) Y wherein Y is a member selected from the chloro and bromo groups, said solution having a pH of about 0.5 to 4 and containing from 5 to 20 grams per liter of ruthenium.

2. A process for electrodepositing ruthenium to provide a hard and Wear resistant surface which comprises cathodically depositing ruthenium from the aqueous acidic bath of claim 1 while maintaining the bath at a temperature in the range from C. to C. and the current density in the range from 0.25 to 10 amperes per square decimeter.

3. A process in accordance with claim 2 in which sulphamic acid or a soluble sulphamate has been added.

4. A process in accordance with claim 3 in which the soluble sulphamate is ammonium sulphamate.

5. A process in accordance with claim 2 in which the pH of the bath is from 1.5 to 2.5.

6. The bath of claim 1 wherein the complex compound of ruthenium has the formula [Ru N(H O) Cl (NH 7. The bath of claim 1 wherein the complex ruthenium compound has the formula [RuN(H O) Br ](NH References Cited UNITED STATES PATENTS 2,600,175 6/ 1952 Volterra 204-47X 3,123,544 3/1964 Blake 204-47 GERALD L. KAPLAN, Primary Examiner US. Cl. X.R. 2350 zgigg i UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3 576 Dated April 27 1 9 71 Inventf(8) GADHIREDDY SATANARAYAN REDDY and PARNI TAIMSALU It is certified that error appears in the above-identified paten and that said Letters Patent are hereby corrected as shown below:

Column 1, line 61, for "as" read has II II Column 3, line 69, for [R1.1 (H O) Br ](NH read-{Ru N (H O) Br (NH Column 5, line 62, for "l..0" read --l.0-

Column 8, line 75, for "CuK" read -Cu K-- Signed and sealed this 14th day of September 1971.

(SEAL) Attest:

EDWARD M.FLETQHER,JR ROBERT GOTTSCHALK attesting offi Actlng Commissioner of Patents 

